THE DESIGN AND TESTING OF AN EXHAUST AIR ENERGY RECOVERY WIND TURBINE GENERATOR

THE DESIGN AND TESTING OF AN EXHAUST AIR ENERGY RECOVERY WIND TURBINE GENERATOR Wen Tong Chong Ahmad Fazlizan Sin Chew Poh Department of Mechanical Engineering Faculty of Engineering University of Malaya 50603 Kuala Lumpur, Malaysia e-mail: chong_wentong@um.edu.my e-mail: afazlizan@yahoo.com e-mail: pohsc@um.edu.my Sook Yee Yip Wooi Ping Hew UMPEDAC, Level 4, Engineering Tower, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia e-mail: arkata_yee@yahoo.com e-mail: wphew@um.edu.my ABSTRACT An innovative system to recover part of the energy from man-made wind resources is introduced. A vertical-axiswind-turbine (VAWT) with an enclosure is mounted above a cooling tower to harness wind energy to generate electricity. The enclosure enhances the performance of the VAWT. It is equipped with guide-vanes that guide the wind to the optimum angle-of-attack of the turbine. Besides, diffuser-plates are mounted at a specific angle to accelerate the airflow. Both laboratory test and on-site field test show no measurable difference on the outlet air speed of the cooling tower; the outlet speed of the cooling tower without and with the VAWT is 10.63 m/s and 10.67 m/s respectively (the rotational speed of the turbine is 881rpm). No difference was observed on the power consumption for both cases (7.0~7.1 kw). This system is retrofit-able to existing cooling towers and has high market potential due to abundant exhaust air resources globally. 1. INTRODUCTION Energy is vital for everything on earth and even more so for the progress of a nation. It has to be conserved in a most efficient manner. The need for a safe, secure and affordable energy supply in both developed and developing countries continues to grow and it may almost double or more by year 2040 which is 30% higher compared to 2010 globally [1]. As a consequence, the environmental impacts of the conventional power supply which relies on fossil fuels and nuclear materials for energy generation are at an alarming condition. In addition to that, these non-renewable energy sources are limited in supply and one day they will be depleted. Therefore, intensive research and development in new energy technologies are essential to keep pace with rising energy demands for a sustainable energy system. Renewable energy with its impressive potential for cost reduction, together with rise in oil prices and the environmental costs linked to the conventional energy supply system make it become an alternative energy source for mass use in the future. Besides, the use of renewable energy resources in combination with very ambitious energy conservation schemes will become increasingly vital for several decades from now [2] because they are better in performance and economic aspect. Over the past 20 years, wind power is reported as the fastest growing renewable energy resource in the world (at the rate of 30% annually) in many countries as it is a clean, cost-effective and sustainable resource [3]. However, Malaysia is located at the equatorial zone and the majority of the areas in the mainland experience low and fluctuating wind speeds throughout the year (freestream wind speed, V <4 m/s for more than 90% of total wind hours) [4]. This climate condition will cause the application of harnessing the natural wind energy by using conventional wind turbine to be inappropriate in Malaysia; an innovative idea was developed as a solution to alleviate this problem which is to extract wind energy from man-made wind resources e.g. exhaust air system or cooling tower. Usually, an exhaust air system provides air that has great potential to be extracted into useful energy because it is higher in wind speed and almost constant in range compared to natural wind. These advantages will result in the installation of wind generators at low wind speed zones such as Malaysia to become feasible. This paper is aimed at presenting a novel energy recovery system from wasted exhaust air. It is done by conducting a number of experimental tests on both actual and fabricated small scaled cooling tower in order to investigate the feasibility of mounting a vertical axis wind 1

turbine (VAWT) with an enclosure on the top of an exhaust air system. The energy recovery system is targeted to produce on-site clean energy generation from the exhaust air system without causing the negative effects on the performance of the original exhaust air system. 2. URBAN BUILDING INTEGRATED WIND TURBINE (BUWT) Wind turbines that are installed within an urban environment (i.e. close to the populated area) are defined as building mounted/integrated wind turbines (BUWTs). They are one of the on-site green building concepts as renewable energy is directly supplied to the building where the energy is required Besides, they have a huge potential in reducing carbon dioxide emissions [5]. Thus, increasing attention has been received over recent years due to its proximity with the point of use. The utilization of wind resources in urban areas is different with conventional open site wind farms. There are many different criteria that need to be addressed during the design and development stages. Structural and architecture of the buildings need to be taken in consideration when choosing the most suitable type of wind turbine to be deployed. In parallel, careful evaluations on power output are conducted to ensure the efficiency and smart use of the energy [6]. Furthermore, factors like position to locate the wind turbine, building orientation, turbulence and wind shadowing effects also should not be overlooked [7]. At this moment, the current utilization types of wind turbine in urban areas are horizontal axis wind turbines (HAWTs) and vertical axis Darrieus and H-rotor type [6] as they are easy to retrofit to the existing building. The world s first large-scale building integrated wind turbine was developed at Bahrain World Trade Center. There are three 29 meter horizontal axis wind turbines incorporated with this 240 meter high building [8]. In London, there is another building with BUWTs which is the Strata Tower. The 3 installed turbines with rated power of 19 kw are estimated to recover 8% of the building electricity demand [9]. Structural and architecture designs of the buildings limit the usage of BUWTs. One of the main reasons is due to the fact that existing wind turbines are unable to adapt to the complex wind environment. High rise buildings tend to have high turbulence. Hence, deploying a turbine on the top of the buildings will need to take this into consideration and it is proven that VAWTs are able to perform better in turbulent wind conditions at high altitude[10]. Muller et al. had presented a vertical axis wind energy converter which is designed with a cylindrical body and it is able to match with current building structural design. In addition to that, Grant et al. also reported that ducted wind turbines that utilized the pressure difference created by the wind flow around the building has the significant potential for retrofitting into the existing building with minimum visual impact [11]. There is also a new development of the wind turbine known as Crossflex which uses the existing Darrieus turbine concept with flexible blade system suggested by Tim Sharpe et al. However, it was applied in a novel form for building integration [12]. Safe and reliable operation of wind turbines in urban areas is specialized and technically challenging. Furthermore, BUWTs add noise pollution and may become an eyesore to the public. These hurdles will drag the progress of integrating wind energy converters architecturally into the building structure. Thus, a new way is required in order to resolve this issue. An innovative idea on implementing a wind turbine generator above an exhaust air system is introduced in this paper. It is an exhaust energy recovery system that is surrounded by an enclosure (equipped with guide vanes and diffuserplates) for better safety security and exhaust air quality to the turbine. Furthermore, this system is easy to retrofit onto any existing exhaust air system without resulting in negative impacts on the original performance of the exhaust air system. 3. PROPOSED DESIGN DESCRIPTION 3.1 Working principles and general arrangement of the design Fig 1: General arrangement of the energy recovery system. This innovative design of an energy recovery system is to reuse the released air from an exhaust outlet as well as natural wind to produce electricity. It is done by mounting two vertical axis wind turbines (VAWTs) that are integrated with an enclosure above the outlet of a cooling tower. Fig. 1 depicts the general arrangement of the designed system. Supporting structure is the main structure for the entire system to hold the VAWTs, guide vanes and enclosure. It can be installed either horizontally or vertically above the exhaust air system depending on 2

the incoming air s direction to the turbine. In order to capture the wind blown from the bottom, the system is installed horizontally with a supporting structure at both ends of the power-transmission shaft of the VAWT with generator at one side and bearing at the other side. In contrary, it is also able to be mounted in a vertical direction with the generator placed on the floor when the exhaust air is blowing from the side. Various types of VAWT that are available from the market can be used in this system. The VAWTs are positioned at a predefined orientation above the exhaust air outlet to ensure zero or minimum negative impact on the performance of an exhaust air system while capturing maximum air flow. Guide vanes are equipped with the enclosure and arranged in-between the exhaust outlet and VAWTs at a predetermined angle. Multiple air flow channels are formed through the guide vane and they are utilized to guide the wind direction to an optimum angle of attack on the VAWT blades. Furthermore, a venturi effect is established and it will greatly help to accelerate exhausted air flow. This feature will promote better self-starting behavior of the VAWTs and they are able to rotate closer to their rated speed. As a result, a large amount of electricity is produced as rotational speed of the VAWTs is enhanced. Diffuser plates are mounted inclined outwardly at an optimum angle relative to the horizontal axis at both sides of the VAWTs. They are capable to collect and accelerate approaching exhausted air flow efficiently. Based on the experiments and studies demonstrated by Abe et al.[13], the performance of diffuser-shrouded wind turbine is better in terms of output power compared to a bare wind turbine as it has about 4 times higher power coefficient. In terms of safety and security concern, the enclosure can act as a safety cover to the wind turbine system. Among the common malfunction or threats that may occur such as blade failure, this can be reduced by adding a mesh around the enclosure. This will protect the system from unwanted flying objects like birds or bats from striking the turbine blades. Further benefits such as minimum acoustic emission from wind turbine or negative visual impacts can also be gained in this manner as the VAWT is installed within the enclosure. time the exhaust air system is operating, the energy generated from the system is predictable and constant in value. With constant rotational speed of turbine, over speed control is not required as only small rotational speed fluctuation is experienced. This feature can increase the longevity of the turbine due to less fatigue experienced. Besides, wind blown by the exhaust s fan has a better quality compared to natural wind. Thus, the statistical analysis of wind characteristic over a period of time is not required before deployment of the turbine. In addition to that, selection of the wind turbine for the system would be simpler because the rated speed of the turbine is based on the exhausted wind and power output generated from the system is easily predictable. Guide vanes are added to the design to guide the air flow to the optimum angle of attack at the wind turbine. The suction effect that occurs as a result from the diffuser plates will cause more air to be drawn into the cooling tower and interact with the wind turbine. This will result in better turbine self-starting behavior as the turbine is able to rotate faster at its rated speed. Moreover, when the VAWTs are rotating at a high rotational speed, the top of the cooling tower will become a low pressure region which is beneficial to assist the fan to discharge air to the outlet faster and thus power consumption of the fan would be decreased. The design of the system takes into accounts the safety of personnel especially when it is planned to be installed in urban areas. The enclosure that is integrated with the wind turbine could act as a safety net to protect the system from unwanted objects that may damage the wind turbine. This energy recovery system has a very high return of investment (ROI) as there are many opportunities to implement this system. Furthermore, this system does not contribute any kind of pollution and it may serve a portion of a nation s demand of electricity. Fig. 2 illustrates an artist impression of the energy recovery system implemented above the cooling tower. 3.1 Benefits of the exhaust air energy recovery system The energy recovery system is designed in a way that the released waste energy from any exhaust air system is converted to useable energy. As the wind from the exhaust air is readily available and concentrated every Fig. 2: Artist s impression of the energy recovery system. 3

Fig. 3: Laboratory testing setup for scaled model of cooling tower. 4. METHODOLOGY Through a series of tests and experiments, the feasibility and the concept of the energy recovery system were demonstrated. These tests included laboratory testing on scaled model and on-site field testing on an actual cooling tower. The effects resulted from the energy recovery system together with wind turbine performance were taken into account as the important parameters for the testing. 4.1 Laboratory testing on the scaled model of cooling tower The laboratory testing on the scaled model of cooling tower was held in the Fluid Mechanics Laboratory, University of Malaya. It was an initial experiment to analyze the feasibility of the designed system. The experiment was carried out as shown in Fig 3. A 5 bladed H-rotor wind turbine with a rotor diameter of 0.3 meter was used in the test. The scaled model of cooling tower was represented by using a 0.7 meter diameter of industrial fan enclosed in a 0.8 meter diameter cylinder duct. Beneath the cooling tower, there was a gap with a distance of 0.195 meter from the floor (with air inlet area of 0.5329m 2 ). Wind turbine was installed in a way that was enclosed within a 0.5 meter diameter of enclosure and positioned at 0.18 meter above the exhaust fan measured from the fan to wind turbine transmission shaft. Diffuser-plates were mounted at both ends of the wind turbine. According to experimental investigation carried out by Abe et al. [10], diffusers are best when inclined at 7 degrees relative to its horizontal axis. The fan speed was set to maximum speed (number 3 on selection buttons). The air will flow through the inlet of the cooling tower (at the bottom gap) and blown out through outlet on top surface and interacts with the turbine. There were three test conditions of experimental testing performed as below:- i. tower without wind turbine ii. tower with wind turbine iii. tower with wind turbine integrated with enclosure Several measurements had been done to identify the difference among the three test conditions. The motor fan s current consumption was measured by using a mini clamp meter at the power cable. Power input, P in to the exhaust air system was calculated with P in = V x I (1) where V is the voltage and I is the electrical current. A hot wire anemometer was used to measure the air intake speed of scaled model of cooling tower at four intake points after the rotational speed of the wind turbine had stabilized. Then, the rotational speed of the wind turbine was measured by a hand held laser tachometer. 4.2 On-site field testing on the actual cooling tower Fig. 4: On-site field testing setup at Truwater Towers Sdn. Bhd. 4

On-site field test was conducted at Truwater Towers Sdn. Bhd. in order to get reliable results on the performance of the energy recovery system. The design and installation of the energy recovery wind turbine generator on the top of the cooling tower is shown in Fig. 4. Two conditions of field testing had been conducted which were:- i. tower without wind turbine ii. tower with wind turbine In this outdoor experimental test, there was a demonstration unit of cooling tower (Model: TXS300-1S) with outer diameter of 2 meters available for field testing. The cooling tower s fan was driven by a 7.5 kw motor. A combination of a 3 bladed Darrieus type VAWT with a rotor diameter of 1.24 meter with 2 layers of Savonius rotor at the center shaft was used in this experiment. An optimum distance which gives the ultimate performance of wind turbine was identified for the motor s shaft according to the measured velocity profile from the outlet. The horizontal distance from the nearest circumference of VAWT to the outlet of the cooling tower was set to half of the diameter of the rotor. The entire system was built on the supporting structure at both ends of the power transmission shaft with the generator at one side and bearing at the other. tower s fan speed was measured by a tachometer pointing on the pulley that was connected to a fan through a belting system as shown in Fig. 5. The rotational speed of the fan is equivalent to the rotational speed of the driven pulley that was connected to the fan via the same shaft. Thus, the rotational speed of the driven pulley was calculated by the pulley speed ratio as follows N 1 =N 2 x (D 2 /D 1 ) (2) where N 1 is the rotational speed of the driven pulley and N 2 is the rotational speed of the driving pulley. D 1 and D 2 are the diameters of the driven pulley and driving pulley respectively. Fig 5: Fan speed measurement on actual cooling tower. The effect of blockage when an object was positioned above the outlet of the cooling tower could be determined by measuring the volume flow rate from the exhaust air outlet with a vane-type anemometer. The volume flow rate was tabulated as below Q = V outlet x A outlet (3) Where Q is the volume flow rate, V outlet is the average exhaust wind speed and A outlet is the outlet area. The wind speed will change from point to point over the area of the exhaust air outlet duct. A suitable measurement method which was developed by Tower Institute (CTI) was adopted to divide the duct area into a few concentric parts of equal region. The wind speed V outlet was calculated by averaging 6 velocities taken at 60 degrees interval around the circle [11]. For example, if the circular duct is divided into 3 concentric parts equal area, 18 points will be measured as shown in Fig. 6. Then, a laser tachometer was pointed at the rotating shaft to measure the wind turbine rotational speed and a 3 phase power meter was used to measure the power consumption by the motor. Fig. 6: Exhaust wind speed measurement method. 5. RESULTS AND DISCUSSION 5.1 Laboratory testing on the scaled model of cooling tower TABLE 1 shows the results obtained from the laboratory tests. The average exhaust wind speed from the scaled model of cooling tower was recorded at 4.15 m/s. Firstly, baseline condition for the conventional cooling tower operation (without energy recovery system installation) was identified as the reference throughout the entire experiment. Based on the measurement, the fan motor consumption was 0.85 ma and the average intake air velocity was 1.97 m/s. After installing 2 VAWTs above the cooling tower, it was observed that the wind turbines were able to self-start and the rotational speed was recorded at 464 rpm. With the integration of diffuser plates with the VAWTs, the rotational speed of the turbines was even faster with the measured value of 501 rpm. This result proves that the design of the diffuser plates is capable of improving the speed of the wind turbines. In addition to that, the air volume flow rate to the cooling tower was increased compared to the conventional cooling tower where it was improved from 1.05 m 3 /s to 1.14 m 3 /s (8.6% increment). This figure shows that the performance of the cooling tower can be greatly improved as more air was drawn into the cooling tower. By referring to the measured current consumption of the fan motor, it could be concluded that the presence of the energy recovery system did not exert extra load on the motor as the current was at 0.85 ma for all 3 cases. 5

TABLE 1: LABORATORY TEST RESULTS Parameter Motor current consumption Motor power consumption Average intake air velocity Intake air flow rate tower without wind turbine tower with wind turbine tower with wind turbine and diffuser 0.85 ma 0.85 ma 0.85 ma 206.37 W 204.38 W 203.78 W 1.97 m/s 2.28 m/s 2.14 m/s 1.05 m 3 /s 1.22 m 3 /s 1.14 m 3 /s Turbine speed - 464 rpm 501 rpm 5.2 On-site field testing on an actual cooling tower On-site field testing is divided into 3 sections. First section was to investigate wind speed profile exhausted from the cooling tower. Then, the effects of the energy recovery system on the cooling tower were studied and lastly the performance of the mounted wind turbine was validated. 5.2.1 Discharge wind speed profile Fig. 7 depicts the tabulated average discharge wind speed measured in five bands on every quarter. From the line trend shown in the measured wind profile, it was observed that the highest discharged wind speed was coming out between band 3 and band 4. However, wind speed was relatively low at band 5 where it was located close to the outer radius. This phenomenon happened because as the discharge air swirled and spread out from the motor fan [14] when it approached the outer radius, the wind was reflected from the inner wall of the cooling tower and caused the reduction in wind speed. Wind speed dropped even further when shifting to band 1(close to the center) due to a belting system located at the center of the cooling tower which blocked the air flow. Based on the results, the VAWTs preferably should be mounted between band 3 and band 4 as the wind speed at this location was the highest. 5.2.2 Performance of cooling tower and wind turbine TABLE 2 shows the summarized results from on-site field testing. Based on equation (3) discussed previously, discharge air speed is proportional to air volume flow rate. Discharge air speed was observed slightly increased by 0.4% after the installation of the energy recovery system above the exhaust air outlet. This increment shows that with the presence of the energy recovery system, more air will be discharged from the cooling tower and hence the performance of the cooling tower was improved. Simultaneously, no negative impact was found on the power consumption of the cooling tower to drive the fan as the measured motor s power consumption was in the range of 7 kw to 7.1 kw for both cases. According to the manufacturer s specification provided for the wind turbine used, the rated rotor rotational speed is 835 rpm and rated power is 300 W. The measured exhaust wind speed from the cooling tower was approximately 10.6 m/s and it was capable to cause the VAWT to rotate at 881 rpm with the condition of no load application. Based on the turbine s power curve, an estimated power of 200 W can be achieved by installing the energy recovery system. TABLE 2: ON-SITE FIELD TEST RESULTS Parameter tower without wind turbine tower with wind turbine Average discharge air speed 10.63 m/s 10.67 m/s Air volume flow rate 33.4 m 3 /s 32.52 m 3 /s tower fan rotational speed 386 rpm 387 rpm Power consumption by the cooling tower 7.0-7.1 kw 7.0-7.1 kw Wind turbine rotational speed - 881 rpm Fig 7: Discharge wind speed profile 6

5.2.3 Estimation of energy generated The power projected to be recovered through the installation of the energy recovery system is covered in this section. Suppose that a 2 meter diameter cooling tower needs 7.5 kwh power and is operated for 16 hours per day, total power consumption for 3000 units of cooling tower will be 131.4 GWh/year. An optimized system with 2 units of wind turbine installed above the cooling tower is capable of producing 1 kwh of energy. As a result, 13% of the power consumption from 3000 units of cooling tower which is equivalent to 17.5 GWh is recovered by implementing the energy recovery system. 6. CONCLUSION An exhaust air energy recovery system without resulting in negative impact is designed. The performance of the VAWTs are efficiently boosted by integrating with an enclosure. Diffusers mounted at an optimum angle created venturi effect and the exhausted air flow was improved. Besides, common safety mishaps such as blade failure problem are eliminated with the installation of the enclosure. It could act as a protective cover to protect the entire system. Due to the simplicity of the design, this energy recovery system is retrofit-able to any existing exhaust air system with minimum visual impact. From the laboratory test conducted, installed VAWT is able to generate electricity without pulling down the original performance of the cooling tower. Instead, the cooling tower s performance was enhanced as air volume flow rate to the cooling tower is raised 8.6% higher. By integrating the VAWT with an enclosure to the design, the VAWT s performance is improved where the rotational speed is raised from 463.72 rpm to 500.98 rpm. On site testing was conducted to further confirm the actual performance and reliability of this system. It was observed that the released air speed from the cooling tower increased by 0.4% more compared to the conventional cooling tower. In terms of the motor s power consumption, there was no significant difference observed for both test cases (7.0~7.1 kw). The implementation of this energy recovery system is possible to conserve power consumption from the cooling tower by up to 13%. By turning what was a wasted energy to useable energy like electricity, this energy recovery system has a high market potential and fast payback period as there are numerous usages of exhaust air system globally. It can be used as supplementary power for building lightings or fed into the electricity grid for energy demand in urban buildings. 7. ACKNOWLEGEMENTS The authors would like to thank University of Malaya for the assistance provided in the patent application of this design (Patent pending: PI 2011700168), and the research grant allocated to further develop this design under the project RG039-09AET (University of Malaya Research Grant) and D000022-16001 (High Impact Research Grant). 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